1. Field of the Invention
The present invention relates to a toner amount measuring apparatus for optically measuring the amount of toner adhering onto a photosensitive medium or transfer belt of an image forming apparatus such as a printer, a copying machine, a facsimile machine, etc., and an image forming apparatus equipped with a toner amount measuring unit having the same function as the toner amount measuring apparatus.
2. Description of the Related Art
A measurement of a toner amount on a photosensitive medium or transfer belt of an image forming apparatus such as a printer, a copying machine, a facsimile machine or the like plays an important role in controlling an image forming process in the image forming apparatus. Therefore, a toner amount measuring apparatus which optically measures the amount of toner has been well known, and also an image forming apparatus in which a toner amount measuring unit having the same function as the toner amount measuring apparatus is installed to control the image forming process on the basis of a measurement result. In the following description, a carrier for toner which is represented by a photosensitive medium, a transfer belt, etc. is generically referred to as xe2x80x9cimage forming bodyxe2x80x9d). Further, in the following description, the terms of the toner amount measuring apparatus and the toner amount measuring unit are used with no discrimination at some places.
Next, the construction of a conventional toner amount measuring apparatus will be described with reference to FIGS. 1 to 4.
The image forming bodies such as the photosensitive medium, the transfer belt, etc. to which toner adheres are generally designed to have a mirror surface structure having high flatness, and the toner amount on these image forming bodies has been hitherto measured by utilizing this surface characteristic.
FIG. 1 is a diagram showing the principle of a toner amount measuring apparatus using mirror reflection.
According to the toner amount measuring apparatus using the mirror reflection, light L1 having a predetermined intensity is emitted from a light source 2 such as a light emitting diode (LED) or the like onto the surface of an image forming body 1, and mirror-reflected as reflected light L2 from the surface of the image forming body 1. The reflected light L2 is detected by an optical sensor 3 such as a photodiode or the like, and a voltage having the magnitude corresponding to the intensity of the reflected light L2 thus detected is output from the optical sensor 3.
The reflected light L2 is intercepted by toner particle 4 in a toner-adhering area on the surface of the image forming body 1. Therefore, the light amount of the light L2 which is reflected from the toner-adhering area on the surface of the image forming body 1 and then detected by the optical sensor 3 is reduced by the amount corresponding to the interception of the light L2, and thus the output voltage from the optical sensor 3 is also reduced.
FIG. 2 is a graph showing the relationship between the amount of toner adhering onto the surface of the image forming body 1 and the output voltage of the optical sensor 3 in the toner amount measuring apparatus using the mirror reflection.
The abscissa of the graph of FIG. 2 represents the amount of toner adhering onto the surface of the image forming body, and the ordinate of the graph of FIG. 2 represents the output voltage of the optical sensor.
As described above, the output voltage of the optical sensor corresponds to the light amount of the mirror-reflection light from the surface of the image forming body. As indicated by a curved line 5 which is drawn from the upper left-hand side to the lower right-hand side in the graph, the output voltage of the optical sensor is reduced as the toner adhering amount increases. By determining the curved line 5 in advance, the amount of toner adhering to the surface of the image forming body can be calculated on the basis of the relationship indicated by the curved line 5 and the output voltage of the optical sensor.
When color toner (color toner particles) is used, light used for irradiation of the color toner particles is diffused due to reflections of the light from the surfaces and inner parts of the color toner particles. A toner amount measuring apparatus using such diffused light has been known.
FIG. 3 is a diagram showing the principle of the toner amount measuring apparatus using the diffused light as described above.
In the case of the toner amount measuring apparatus using the diffused light, light L1 having a predetermined intensity is emitted from a light source 2 to the surface of the image forming body 1 as in the case of the toner amount measuring apparatus using the reflected light. However, an optical sensor 6 is disposed at a position out of the travel path of the reflected light L2 shown in FIG. 1. The diffused light L3 caused by the toner particle 4 adhering to the surface of the image forming body 1 is detected by the optical sensor 6, and the voltage corresponding to the intensity of the diffused light L3 thus detected is output from the optical sensor 6.
FIG. 4 is a graph showing the relationship between the toner adhering amount and the output voltage of the optical sensor in the toner amount measuring apparatus using the diffused light.
Like the graph of FIG. 2, the abscissa of the graph of FIG. 4 represents the toner amount, and the ordinate thereof represents the output voltage of the optical sensor. In this case, the output voltage of the optical sensor corresponds to the light amount of the diffused light caused by the toner 4.
As indicated by a curved line 7 in the graph of FIG. 4, the output voltage of the optical sensor increases as the toner adhering amount increases. By determining such a curved line 7 in advance, the amount of toner adhering to the surface of the image forming body can be calculated on the basis of the relationship indicated by the curved line 7 and the output voltage of the optical sensor.
Most of conventional toner amount measuring apparatuses use only one or both of the measuring principles shown in FIGS. 1 and 3, and for example Japanese Patent Laid-open No. Hei-6-66722 discloses one of these toner amount measuring apparatuses.
Here, a target for the toner amount measurement will be described.
FIG. 5 shows a target (object) for the toner amount measurement. In the following description, a toner-amount measurement target in an electrophotographic image forming apparatus will be described.
A toner image is formed according to the procedure described below in the electrophotographic image forming apparatus.
First, the surface of a photosensitive roll 8 rotating in the direction indicated by an arrow F of FIG. 5 is uniformly charged by a bias charging unit 9, and then the surface of the photosensitive roll 8 is irradiated with a laser beam emitted by a laser exposing unit 10 to form an electrostatic latent image. Subsequently, toner adheres to the electrostatic latent image with a developing unit 11 to form a toner image 12. The toner image 12 thus formed is transferred onto a transfer belt 14 by a transferring unit 13 to form a transfer image 15. The transfer image 15 is subsequently transferred to a sheet again, and finally a toner image is formed on the sheet.
The toner image 12 on the photosensitive roll 8 and the transfer image 15 on the transfer belt 14 have been targeted as toner-amount measurement objects, and the amount of toner constituting the toner image 12 and the transfer image 15 is set to about 0.1 to 0.7 mg/cm2. The toner amount in this range can be measured with high precision by using the conventional toner amount measuring apparatuses.
In addition to the toner with which the toner image 12 and the transfer image 15 as described above are formed, a minute amount of toner which induces xe2x80x9cfogxe2x80x9d or xe2x80x9cresidual tonerxe2x80x9d as described later is also particularly targeted as an object on a photosensitive roll 8 or the like on which the toner amount measurement is conducted.
The xe2x80x9cfogxe2x80x9d is caused by toner which adheres to a background portion to which the toner should be originally avoided from adhering in a process of making the toner adhere to an electrostatic latent image with the developing unit 11 as described above. Therefore, if the amount of toner which induces xe2x80x9cfogxe2x80x9d is large, the background of an image on a sheet would finally become blackish uniformly or colored.
Next, the xe2x80x9cresidual tonerxe2x80x9d 16 will be described.
As described above, the toner image 12 on the photosensitive roll 8 is transferred to the transfer belt 14 to form the transfer image 15. In this transfer process, the transfer efficiency of toner from the photosensitive roll 8 to the transfer belt 14 is not equal to 100%, but equal to about 99% at maximum. This means that a part of the toner constituting the toner image 12 on the photosensitive roll 8 remains on the photosensitive roll 8. The toner remaining on the photosensitive roll 8 means the xe2x80x9cresidual tonerxe2x80x9d 16. The xe2x80x9cresidual tonerxe2x80x9d 16 is usually removed by a cleaner 17. However, as the toner amount of the xe2x80x9cresidual tonerxe2x80x9d 16 increases, cleaning failure occurs and thus image quality is lowered,
The toner amount of xe2x80x9cfogxe2x80x9d and xe2x80x9cresidual tonerxe2x80x9d is equal to 0.01 mg/cm2 or less. Accordingly, a toner amount measuring apparatus which measures such a minute amount of toner is required to have such a specification that the measurable range covers a minute toner-amount area from 0.0004 mg/cm2 to 0.01 mg/cm2, and the measuring precision in this toner-amount area is equal to about 0.0004 mg/cm2.
Next, the measuring performance for such a minute toner-amount area in the conventional toner amount measuring apparatus will be described.
FIG. 6 is a graph showing the relationship between the toner amount and the output voltage for a minute toner-amount area in the conventional toner-amount measuring apparatus using the mirror reflection.
The ordinate of the graph of FIG. 6 represents the toner amount of xe2x80x9cresidual tonerxe2x80x9d. The toner amount of xe2x80x9cresidual tonerxe2x80x9d was determined by a method of enlarging toner particles in the minute toner-amount area with a microscope and counting the number of the toner particles or other methods. The abscissa of the graph of FIG. 6 represents a relative output voltage of the toner amount measuring apparatus when the output voltage for the toner amount of xe2x80x9c0xe2x80x9d is set as a reference value.
From the viewpoint of the overall dynamic range of the optical sensor equipped in the toner amount measuring apparatus, it is indicated that the output voltage of the toner amount measuring apparatus is substantially saturated at a toner amount of about 0.5 mg/cm2. However, paying attention to the relative output voltage as described above, the output voltage is substantially proportional to the toner amount in the minute toner-amount area as indicated by a straight line 18 of the graph shown in FIG. 6. This relationship between the output voltage and the toner amount is obtained by repeating the measurement at many times, and the output voltage data obtained for the minute toner-amount area has a large dispersion as indicated by the distribution of marks 19 which indicate respective measurement results. Therefore, the measurement precision approximately ranges from 0.002 to 0.004 mg/cm2, and thus it is lower than the required measurement precision described above by about one order.
Next, noises in the measurement of a minute amount of toner will be described.
FIGS. 7 to 9 are graphs showing measurement data obtained by using the conventional toner amount measuring apparatus.
These data were obtained by making measurements on the surface of a photosensitive roll rotating at a predetermined rotational frequency. FIG. 7 is a graph showing data under the state that toner was perfectly removed, FIG. 8 is a graph showing data under the state that toner adhered to the surface of the photosensitive roll and FIG. 9 is a graph showing the difference between the data of FIG. 7 and the data of FIG. 8. The abscissas in FIGS. 7 to 9 represent the time, and the ordinates in FIGS. 7 to 9 represent the output voltage.
Waviness 20 having a large period which exists on the graphs of FIGS. 7 and 8 corresponds to a stationary variation which is caused by dispersion in the surface shape or reflectivity of the photosensitive roll. Further, a large number of fine peaks 21 occur on the graphs of FIGS. 7 and 8. These peaks are caused by electromagnetic noises occurring in the image forming apparatus, and a high-voltage source and a bias charging unit may be considered as sources which generate these electromagnetic noises.
An upper line 22 of two fine lines 22, 23 in FIG. 9 indicates data obtained by subjecting the data of FIG. 7 to 25-times movement average processing to remove the peaks 21 based on the electromagnetic noises. Likewise, the lower line 23 of the two fine lines 22, 23 indicates data obtained on the basis of the data of FIG. 8.
A heavy line 24 of FIG. 9 indicates data by matching the phase between the data indicated by the fine lines 22, 23 and then conducting subtraction processing on the data thus phase-matched, and peaks 25 appear at toner-adhering portions.
The height of the peak 25 has sufficient reproducibility for the same measurement target. However, when the measurement target is varied, the peak height is varied even for the same toner amount, and thus it has a large dispersion between data. Therefore, it is difficult to achieve the measurement precision in the specification required as described above.
As described above, with respect to the conventional toner amount measuring apparatus, it is difficult to control the image forming conditions by measuring xe2x80x9cfogxe2x80x9d and xe2x80x9cresidual tonerxe2x80x9d with the conventional toner amount measuring apparatus because the measurement precision for the minute toner-amount area is low.
Further, when the toner amount of xe2x80x9cfogxe2x80x9d or xe2x80x9cresidual tonerxe2x80x9d is required to be measured in the developing process of an image forming apparatus, there has been hitherto used such a method that the image forming apparatus is stopped to detach from the image forming apparatus a photosensitive roll to which toner of xe2x80x9cfogxe2x80x9d or xe2x80x9cresidual tonerxe2x80x9d adheres, and then the toner is enlarged with a microscope or the like to count the number of toner particles. However, the measurement of the toner amount by using this method needs a large number of steps to merely perform the measurement, and much developing time and a high developing cost are needed for the image forming apparatus.
The present invention has been made in view of the above circumstances, and provides a toner amount measuring apparatus which can measure a minute amount of toner inducing xe2x80x9cresidual tonerxe2x80x9d and xe2x80x9cfogxe2x80x9d with high precision, and an image forming apparatus equipped with a toner amount measuring unit having the same function as the toner amount measuring apparatus to thereby form a high-quality image.
According to an aspect of the present invention, a toner amount measuring apparatus which detects reflection light reflected from an image forming body carrying toner to measure the amount of the toner carried on the image forming body, is characterized by including: an irradiating unit which irradiates the image forming body with light to form fringes of light on the image forming body; and a photodetecting unit which detects reflection light obtained by reflecting the light forming the fringes from the image forming body.
Further, according to another aspect of the present invention, an image forming apparatus which finally forms a toner image on a sheet under a controllable image forming condition, is characterized by including: an image forming body which carries toner to form the toner image; a toner amount measuring unit which is equipped with an irradiating unit for irradiating the image forming body with light to form fringes of light on the image forming body and a photodetecting unit for detecting reflection light obtained by reflecting the light forming the fringes from the image forming body, and measures the amount of toner carried on the image forming body; and a condition controlling unit for controlling the image forming condition on the basis of the toner amount measured by the toner amount measuring unit.
According to another aspect of the present invention, a toner amount measuring method has the steps of: irradiating an image forming body with light to form fringes of light thereon; detecting the light forming the fringes reflected from the image forming body and outputting a photodetection signal in accordance with the amount of the light thus detected; and extracting a signal component having a specific frequency from the photodetection signal.